Direct intravital microscopy is used to characterize changes in the behavior of the microcirculation associated with the development of hypertension in the spinotrapezius muscle of rats. Emphasis is placed on changes in segmental and overall resistance to blood flow during the evolution of the syndrome in a special strain of rats (so called SHR). On-line measurements were made of vessel diameter, red cell velocity and micropressure levels in representative segments of the microvascular tree beginning with 50-75 micrometer arterioles, through the capillaries and the effluent venules (70-90 micrometers). Pressure-flow relations in hypertensives show that resistance for all of the microvessels on the arterial side of the network is elevated with the maximal difference occurring in the 10-20 micrometer precapillaries. The evidence indicates that vascular smooth muscle tone as well as structural changes are involved appearing as early as at 4 weeks of age and becoming progressively more severe at 16-18 weeks of age. Resistance for the capillary network proper is lower in hypertensives. Venular resistance is not significantly affected. The markedly elevated increase in precapillary resistance in hypertensives is associated with a reduction in the number of perfused capillaries.